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Question:

Structure of a mixed oxide is cubic close-packed (ccp). The cubic unit cell of mixed oxide is composed of oxide ions. One-fourth of the tetrahedral voids are occupied by divalent metal A and the octahedral voids are occupied by a monovalent metal B. The formula of the oxide is:

A2BO2

A2B3O4

ABO2

AB2O2

Solution:

It is given that the structure of mixed oxide is cubic closed packed.
Therefore, number of O2- ions according to ccp = 4
So, number of tetrahedral voids = 8
and octahedral voids = 4
A ions occupy 1/4th of tetrahedral voids = 2
B ions occupy all octahedral voids = 4
A:B:O = 2:4:4 = 1:2:2
Hence, the formula is AB2O2.